Fractures in rock masses influence strongly the mechanical and hydraulic properties of a rock mass. Thus, the detection and characterization of fractures using geophysical methods is of critical importance for maintaining the integrity of sub-surface infrastructure and subsurface waste or storage repositories. While the effects of single fractures or sets of parallel fractures on seismic wave propagation have been studied by many scientists and engineers, little research has been performed to determine the role of fracture intersections on seismic wave attenuation and velocity. A fundamental question is whether the specific stiffness or compliance of an intersection is the same or differs from the stiffness of any of the individual fractures within two intersecting sets of fractures. In this paper, we show from experimental and numerical studies that the stiffness of fracture intersections can be less than, equal to, or greater than the stiffness of the individual fractures depending on the applied bi-axial loading conditions.

1. INTRODUCTION

Rock fractures often occur in sets, as networks, or singly on nearly all length scales. Although many fracture sets contain parallel fractures, sets with orthogonal intersecting fractures are also common. Orthogonal fractures are prevalent in many geologic formations found in Norway, the United States, the United Kingdom, and even on extraterrestrial bodies such as the Moon [1-4]. Extensive theoretical, computational and experimental research has examined the effect of single fractures and fracture sets in isotropic and anisotropic media on seismic wave propagation [5-13]. Hydraulic studies on similar fractures have also been conducted as well as for intersecting fractures [14-23]. It is surprising then, that so little work has been done on characterizing orthogonal fracture intersections seismically. From the aforementioned studies on single and parallel sets of fractures, fractures give rise to converted modes, guided modes, and anisotropy.